Windows 7 Most Hit by WannaCry Ransomware

Most of the computers affected by the WannaCry ransomware outbreak were running Windows 7, security researchers have revealed.

Initially, the malware was believed to have hit mostly computers running Windows XP, mainly because of its attack vector – exploiting a Server Message Block (SMB) version 1 vulnerability. According to a tweet from Kaspersky Lab’s director of Global Research and Analysis Team Costin Raiu, however, the number of Windows XP infections was insignificant.

Windows 7 x64 machines were hit the most, accounting for 60.35% of infections, with Windows 7 x86 coming in second, at 31.72%, the researcher also revealed. These two Windows 7 versions, along with Windows 7 Home x64 and x86 editions, accounted for around 98% of all WannaCry infections, it seems.

WannaCry made a name for itself after researchers discovered it had a worm component abusing the NSA-linked EternalBlue and DoublePulsar exploits to automatically spread to other vulnerable machines. The exploit was said to target all Windows versions from XP to 8.1 (Windows Server 2003 & 2008 as well), but the worm is now said to be reliable only when hitting Windows 7.

Actual infection numbers aren’t out yet, but researchers estimate that around 420,000 machines have been hit by the ransomware to date. Because a researcher registered a kill-switch domain soon after the outbreak started (upon infection, the malware would beacon to a hardcoded domain and terminate its process when receiving a response), only some of these machines ended up infected with WannaCry.

WannaCry hasn’t infected only PCs, but other types of machines as well, including medical devices. In fact, Britain’s National Health Service (NHS) was among the first organizations to have been hit by the malware.

Soon after the initial wave of infections, security researchers started observing new WannaCry variations, including some that didn’t use a kill-switch domain. What’s more, Cyphort researchers reported last week that a new ransomware variant was using a kill-switch domain that couldn’t be registered.

The variant uses a domain in the .test Top Level Domain, which cannot be registered, as it is reserved by the IETF (Internet Engineering Task Force) for testing purposes only, Cyphort says. Because the sample has been submitted to VirusTotal from 4 different countries (Germany, Australia, Denmark and South Korea), it’s unlikely that it is a test.

“It seems that the cyber criminals found a smarter way to evade sandbox detection by checking on a site that researchers cannot sinkhole. This technique allows the malware to spread again unchallenged. It is crucial that people patch Windows machines as soon as possible to close the SMB vulnerability and stop the spread of this ransomware. In the meantime, make sure you have a good backup of your important files,” Cyphort says.

In the meantime, security researchers are working on tools that can help WannaCry victims recover their files without paying the ransom. One of them is Wannakey, designed to extract key material from infected Windows XP PCs. However, it requires a second tool to decrypt files.

Building on Wannakey and already tested by Europol, a tool called wanakiwi appears more suited for the file decryption/restoration operation. One thing that both tools require, however, is that the WannaCry-infected computers haven’t been rebooted after the encryption took place. Already confirmed to work on Windows XP, 7, and Server 2003 (x86), wanakiwi might also work on Vista and Server 2008 and 2008 R2.